A Power-on-Reset circuit or POR circuit is generally used to issue a digital POR signal with respect to the supply voltage level of the circuitry in order to release or reset digital circuitry. If the supply voltage level is below a critical voltage level the digital circuitry is reset. If the critical voltage level is exceeded, the circuitry is released. Therefore, the POR circuitry monitors the supply voltage level and generates the POR signal as function of the monitored supply voltage level. Conventional POR circuits, such as circuit 100 of FIG. 1, has some disadvantages which become more relevant as voltage supply levels are reduced and tolerance margins are more limited. The rather simple circuit 100 comprises a resistor R0 that is coupled to a current mirror (which is generally comprised of diode-connect NMOS transistor NM1 and NMOS transistor NM2). Transistor NM2 is also coupled to a diode-connected PMOS transistor PM1. Transistors NM1 and PM1 are coupled at their drains to a CMOS inverter (which is generally comprised of NMOS transistor NM3 and PMOS transistor PM2) that provides the POR output signal PORout. The POR output signal PORout is typically used as a reset signal, which assumes a logic high or logic low level as function of the supply voltage level VDD. The supply voltage level VDD, at which the PORout signal changes state, is referred to as trigger voltage level. The trigger voltage level is defined by the dimensions (e.g., aspect ratios i.e. the width to length ratio (W/L ratio) of transistors, resistance of R0, and so forth). The trigger voltage level will typically depend on the threshold voltage levels of the MOSFET transistors as well. The properties of the components (as well as the whole circuit) are a subject to temperature and process variations, however. Therefore, the trigger voltage level of a conventional POR circuit, such as circuit 100, usually has a wide spread or variance (e.g., several hundreds millivolts). Furthermore, the trigger voltage level can not be set to an arbitrary value during operation.
Some examples of conventional circuits are: U.S. Patent Pre-Grant Publ. No. 2005/0140406, and U.S. Pat. Nos. 5,959,477; 6,137,324; 6,144,238; 6,239,630; and 7,417,476.